1. Field of Invention
The present invention relates to fuel injectors and, more specifically, to an electromagnetic fuel injector with a braking device.
2. Description of Related Art
Electromagnetic fuel injectors known in the art (for example, the type described in European patent application EP1619384A2) generally include a cylindrical tubular body having a central feeding channel (which has a fuel conveying function) and ends with an injection nozzle adjusted by an injection valve controlled by an electromagnetic actuator. The injection valve includes a needle, which is moved by the action of the electromagnetic actuator between a closing position and an opening position of the injection nozzle against the bias of a closing spring, which tends to hold the needle in the closing position. The electromagnetic actuator is typically provided with a closing spring which pushes the needle towards the closing position, and with an electromagnet which pushes the needle towards the opening position against the elastic bias generated by the spring.
The electromagnet includes a coil externally arranged in a fixed position about the tubular body, a movable plunger firmly connected to the needle and movably mounted inside the tubular body, and a fixed magnetic pole made of ferromagnetic material; arranged within the tubular body at the coil; and adapted to magnetically attract the plunger. The magnetic pole is centrally perforated and has a central through hole which allows the fuel to flow towards the injection nozzle. The closing spring is arranged inside the central hole and is compressed between a perforated catch body driven into the central hole, and the plunger, so as to push the plunger (and, thus, the needle integral with the plunger) towards the closing position of the injection nozzle.
The manufacturers of Otto cycle heat engines (for example, spark-ignition internal combustion engines) often require increased fuel pressure (even in excess of 50 MPa) so as to improve the mixing of fuel, to support combustion and reduce the generation of black smoke (which indicates poor combustion), and to increase the dynamic performance of the electromagnetic injectors (for example, to increase the response speed of the electromagnetic injectors to commands) in order to inject small amounts of fuel with the goal of fractioning the fuel injection into multiple separate injections, whereby the generation of polluting substances during combustion can be reduced.
In an electromagnetic fuel injector, increasing the fuel feeding pressure causes a proportional increase of the hydraulic forces involved, and thus necessitates the use of stronger closing springs and more powerful electromagnets. In order to increase the power of an electromagnet (for example, to increase the magnetic attraction force generated by the electromagnet), either higher performance materials can be used (but, with a considerable increase in costs which is not normally acceptable by the modern automotive industry), or the size of the electromagnet can be increased. Regardless, an increase in the electromagnet size also causes an increase of the magnetic and mechanical inertia of the electromagnet, which then becomes slower. Specifically, increasing the size of the electromagnet inevitably degrades the dynamic performance of the electromagnet itself.
In order to obtain an increase in the force generated by the electromagnet without degrading the dynamic performance of the electromagnet itself, European patent EP1650428B1 suggests doubling the electromagnet; for example, two small-sized twin electromagnets are used instead of a single large-sized electromagnet.
When the injection valve is closed, there is a force of hydraulic nature which pushes on the shutter and maintains the shutter in the closing position (for example, the higher the fuel feeding pressure, the higher this force). Therefore, in order to open the injection valve, the electromagnetic actuator needs to generate a force on the needle which overcomes the force added to the elastic bias exerted by the closing spring. However, the force suddenly disappears as soon as the injection valve opens, thus the injection valve opens very quickly and violently with an extremely fast movement of the needle. Such a fast, violent opening of the injection valve causes a very steep and often irregular ramp in the initial part (referred to as the “ballistic zone”) of the injection law of the injector (for example, the law which relates the actuation time to the injected fuel amount; for example, the driving time).
Because the initial part of the injection law has a very steep and often irregular ramp, correctly controlling the fuel injection is very complex. Moreover, at such a steep ramp, tiny differences in the injection time (for example, in the control time) determine substantial differences in the injected fuel amount.